Coincidence prism



July 30, 1929.

G. W. MOFFITT COINCIDENCE PRISMl Filed-Feb- 24, 1926 2 Sheets-Sheet SNCM mm1 July 30, 1929. v G, w, MOFFlTT 1,722,964

COINGIDENCE RIsM Filed Feb. 24, 1926 2 Sheets-Sheet 2 Eenrge' W. Mnfftt Patented July 30, i929.

GEORGE w. Morrrrr, or PHILADELPHIA, PENNSYLVANIA.

CGNCIDENCE PRSEYZ.

Application fue@ Een-nary ieee. serial No. 90,435.

(GRANTED UNDER THE E39,VISINS` QF THE AST QF MARCH 3, 1883, AS AMENBED APRIL 30, 1928;

The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment to me of any royalty thereon.

This invention relates to coincidence prisms especially applicable to base telemeters though not restricted to this class of instruments. f

ln short base telemeters designed for military purposes it is highly desirable that the instrument be of such outside diameter that it may be readily picked up and carried with one hand and this determination calls for a compactly arranged coincidence prism. Wlith the type of coincidence prisms heretofore used, this condition can not be fulfilled without decentering the optical tube or resorting` to mechanical design which would weaken the instrument and make assembly and repair a diilicult task.

The principal object of the invention then is to provide a compact coincidence prism which may be entirely mounted in an optica-l tube of small diameter without decentering the tube or imposing undesirable mechanical construction and while n retaini the optical centers of the receiving prisms in coincidence with the optical axis of the telemeter.

Another object is to provide a simple and easily constructed coincidence prism in which the base prism which is susceptible for mounting the unit carries two smaller receiving prisms and serves to deflect the beam from one of these prisms into the other prism whose emergent face is so disposed that the ocular associated therewith may be l inclined for convenience in observation.

To these and other ends,rmy invention consists in the construction, arrangement, and combination of elements, described hereinafter and pointed out in the claims forming a part of this specification. A practical embodiment of my invention is illustrated in the accompanying` drawings, wherein Fig. l is a transverse sectional view through the optical tube of a telemeter showing the disposition of my improved coincidence prism;

Fig. 2 is a perspective of the prism as viewed normally to the emergent face;

Figs. 3 to 6 are perspectives of the prism as viewed from above;

Figs. 7 to l() are elevational. views of the prism;

llig. ll is a top plan of the base reflecting prism;

F ig. l2 is a view of the prism system separated and showing the paths of the light rays.

Referring to the drawings by characters of reference VThe prism system comprises two reflecting receiving prisms A and B disposed in the optical axis of the tube T and attached by means of one of their sides on the upper face of a third and larger base reilecting prism@ whose iorm and disposition in the tube determines the diameter of the tube. The base prism is formed with a view towards mounting of the system and accordingly, the portions designated by the circles l0 on the upper face indicate the areas of contact of the prism with the referencesurface of its metal holder, the prism being' held against this reference seat by spring action applied to the bottom face of` the prism, and its three corners il ground to receive positioning screws for holding it against projections touching the face Ll near its ends to prevent lateral dispiacement of the prism on its seat.

It will be noted by referring to Fig. 2 that the longest side or edge of the base prism is oblique to the axis of the tube; and this disposition is made so that the optical centers of both prisms A and B may be in coincidence with the optical axis of the telemeter while still providing ample attaching surface for said prisms without extending laterally beyond the outermost edge of the prism B whose radial distance OP F ig. l) is the ultimate factor governing the size of the optical tube T.

The oblong area designated by l2 on the emergence face 5 of the prism B indicates the projection of the lield stop oft-he instrument5 the portion beneath the dividing line 13 containing an erect image andthe upper portion an inverted image. rlhe paths of the light rays which are collected by the system of prisms and directed to the ocular will be made clear by following the course of the axial ray from the objective lens at each end of the base telemeter.

The ray from the right objective lens is incident on the face 1 of t-he receiving prism A and being normal to this surface it enters the prism without refraction and is totally reflected 'downward in a transverse vertical plane by the face 2, but making an angle of 9O in one of the best forms, until it strikes the face 3 of the base prism. The ray is then successively totally reflected'by the faces 3 and 4 and then reflected by the face 5l upward at an angle passing the edge of the silvered strip 9 on the lower face' of the prism B and finally emerging from the face 6 of that prism ina direction normal to that face. The light that takes the course described forms a fully erected image in the lower portion of the field of View, the edge of the silvere'd area 9 forming'the dividing line 13 in the field.

Considering the left objective of the instru! ment, the axial ray .enters the face 7 of the receiving prism B undeviated' and; is succes# sivelytotally' reflected -by the facesS and 6 to the silvered area 9. From the silver,`the

ray is reflected and 'emerges normalto the face G. Light taking this course forms an inverted but not 'ai reversed image in the upper portion of the field of View.

It Will'be noted that by reason of the disposition of the component risms vand the slightly inclined mounting o the system that the emergent beams are directed into the ocular at a very convenient angle for observation.

By referring to Fig.` l it Will benotedthat the under faces of the base prism conform roughly to the contour of the optical tube and Vthis in conjunction with the functions per f formed by the prism lB by which its radial" distance is kept small is favorable to placing the system entirely Withinlan optical tubetube, a coincidence prism entirely Within said tube, said prism comprising an obliquely positioned large reflecting base prism and two smaller-receiving prisms having undivided 'smaller'prisms being coincident with the optical axisA of the tube.

2. A base telemeter including an optical tube,"a coincidence 'prism Within said tube, said prism comprising a single base reflecting prism and two receiving reflecting prisms mounted on the upper face of said base prism, the ioptical `centersof said-receiving prisms being coincident with the optical axis' of the tube.

3. A coincidence prism systemcomprising a large base prism 'havir'ig 'threereflecting 'surfaces and two smaller-receiving prisms' mounted on the base prismandhaving coprisms being the Acollecting 'pris'mand having three reflecting surfaces presenting in emergence an inverted image :and the other receiving prism being a right angle prism'and cooperating With the vreflecting surfaces of the base prism in presenting an erect and unreversed image emergent through the collect,- ing prism. Y Y

4. A coincidence prism system' comprising a large base reflecting prism and tivo smaller receiving prisms mounted 1on t-he base prism v"axial optical centers, one of saidreceiving and having co-axial optical centers, said re ceiving'prisms having undivided reflecting surfaces.

5. A coincidence prism comprising a re- 5 fleeting receiving prism having an eniergent optical center co-a-Xial With the center of the first prism, and a prism'having reflecting surfaces for directing the image reflected by the second prism through the emergent face of the first prism. Y'

6. A coincidence prism comprising la receiving prism having reflecting surfaces presenting an inverted image by reflection Within itself, a second right angle receiving prism cc-axial With the first prism, and a prism having reflecting surfaces cooperating with the right angle prism in directing an erect and unreversed image through the first prism and parallel with its image.

GEORGE W. MOF F ITT.

second reflecting receiving prism having its i 

